Gallium nitride compound semiconductor device and manufacturing method

ABSTRACT

A light-emitting element using GaN. On a substrate ( 10 ), formed are an SiN buffer layer ( 12 ), a GaN buffer layer ( 14 ), an undoped GaN layer ( 16 ), an Si-doped n-GaN layer ( 18 ), an SLS layer ( 20 ), an undoped GaN layer ( 22 ), an MQW light-emitting layer ( 24 ), an SLS layer ( 26 ), and a p-GaN layer ( 28 ), forming a p electrode ( 30 ) and an n electrode ( 32 ). The MQW light-emitting layer ( 24 ) has a structure in which InGaN well layers and AlGaN barrier layers are alternated. The Al content ratios of the SLS layers ( 20 , and  26 ) are more than 5% and less than 24%. The In content ratio of the well layer in the MQW light-emitting layer ( 24 ) is more than 3% and less than 20%. The Al content ratio of the barrier layer is more than 1% and less than 30%. By adjusting the content ratio and film thickness of each layer to a desired value, the light luminous efficiency for wavelength of less than 400 nm is improved.

TECHNICAL FIELD

The present invention relates to a gallium nitride (GaN)-based compoundsemiconductor device and a manufacturing method thereof, and, inparticular, to improvement in light emission efficiency.

BACKGROUND ART

Light emitting elements which use a nitride semiconductor and have awavelength band of 370 nm-550 nm have been made commercially available.In these light emitting elements, In_(x)Ga_(1-x)N (0<x<1) is used as alight emitting material. The light emission wavelength changes when acompositional ratio of In in In_(x)Ga_(1-x)N is changed. Specifically,the light emission wavelength is increased as x is increased. When thecompositional ratio x of In is changed, the light emission efficiencyalso changes in addition to the light emission wavelength. Morespecifically, when the compositional ratio x of In is increased toomuch, because (1) a difference in lattice constants between InGaN andthe layers sandwiching InGaN, that is, between InGaN and GaN or betweenInGaN and AlGaN, becomes large and (2) a crystal growth temperature mustbe reduced in order to grow crystal of InGaN having a high compositionof In, the crystal quality of InGaN is degraded and the light emissionefficiency is reduced when the wavelength exceeds 530 nm. In general, ina wavelength range of 400 nm-530 nm, the light emission efficiency isincreased, but the light emission efficiency is reduced when thewavelength is 400 nm or smaller.

A reason why the light emission efficiency at a shorter wavelength sideof 400 nm or smaller is reduced may be considered to be due todislocations present in the crystal. An efficiency of a light emittingelement (LED or the like) having a wavelength of 400 nm-530 nm having anarbitrary compositional ratio of In is high regardless of a dislocationdensity because of a fluctuation of In composition within the InGaNlayer. More specifically, when there is a compositional fluctuation ofIn, light is emitted at a local region in which the In composition islarge and the injected carriers are captured in this local region. As aconsequence, the carriers do not reach the dislocations and theefficiency is not reduced. As described above, in order to shorten thelight emission wavelength, the compositional ratio x of In must bereduced, which inevitably results in reduction in the fluctuation of Incomposition. When the fluctuation in composition is small, the carriersare not sufficiently captured. As a consequence, the carriers reach thedislocations and the light emission efficiency is reduced.

As described, when the light emission wavelength is 400 nm or smaller,the light emission efficiency significantly depends on the dislocationdensity and is reduced due to the presence of dislocations.

In order to prevent reduction of light emission efficiency in awavelength band of 400 nm or smaller, the dislocation density must bemaintained at a low level. The dislocation density is reduced in therelated art, for example, through an ELO (Epitaxial Lateral Overgrowth)method and a method in which a light emitting layer is grown on asapphire substrate or the like onto which a groove is formed. However,because these methods require photolithography or the like, there hadbeen a problem in that the manufacturing requires labor and the cost ofthe light emitting element is increased.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a device having asuperior light emission efficiency in a short wavelength range (inparticular, in a wavelength range of 400 nm or shorter) without using aspecial method such as photolithography.

According to one aspect of the present invention, there is provided agallium nitride-based compound semiconductor device comprising asubstrate, a first superlattice layer which is formed above thesubstrate and in which an n-type AlGaN layer and an n-type GaN layer arealternately layered, a multiple quantum well layer which is formed abovethe first superlattice layer and in which a GaN-based quantum well layerand a GaN-based quantum barrier layer are alternately layered, and asecond superlattice layer which is formed above the multiple quantumwell layer and in which a p-type AlGaN layer and a p-type GaN layer arealternately layered.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, abuffer layer, a first GaN-based layer, and an n-type GaN-based layer areprovided between the substrate and the first superlattice layer, asecond GaN-based layer is provided above the first superlattice layer,and a p-type GaN-based layer is provided above the second superlatticelayer.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, acompositional ratio of Al in the GaN-based quantum barrier layer in themultiple quantum well layer is larger than compositional ratios of Al inthe first superlattice layer and the second superlattice layer.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, eachof compositional ratios of Al in the AlGaN layer in the firstsuperlattice layer and in the second superlattice layer is 5% or greaterand 25% or smaller, a compositional ratio of In in the InGaN quantumwell layer or the AlInGaN quantum well layer in the multiple quantumwell layer is 3% or greater and 20% or smaller, a compositional ratio ofAl in the AlGaN quantum barrier layer or the AlInGaN quantum barrierlayer in the multiple quantum well layer is 1% or greater and 30% orsmaller, and a band gap of the quantum well layer is smaller than a bandgap of the quantum barrier layer.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, eachof thicknesses of the AlGaN layer and the GaN layer in the firstsuperlattice layer is 1 nm or greater and 10 nm or smaller, a thicknessof the quantum well layer in the multiple quantum well layer is 1 nm orgreater and 5 nm or smaller, a thickness of the quantum barrier layer inthe multiple quantum well layer is 2 nm or greater and 50 nm or smaller,a thickness of the AlGaN layer in the second superlattice layer is 0.5nm or greater and 10 nm or smaller, and a thickness of the GaN layer inthe second superlattice layer is 0.5 nm or greater and 5 nm or smaller.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, athickness of the first GaN-based layer is 500 nm or greater and 3000 nmor smaller, a thickness of the n-type GaN-based layer is 500 nm orgreater and 10000 nm or smaller, each of thicknesses of the AlGaN layerand the GaN layer in the first superlattice layer is 1 nm or greater and10 nm or smaller, a thickness of the second GaN-based layer is 5 nm orgreater and 100 nm or smaller, a thickness of the quantum well layer inthe multiple quantum well layer is 1 nm or greater and 5 nm or smaller,a thickness of the quantum barrier layer in the multiple quantum welllayer is 2 nm or greater and 50 nm or smaller, a thickness of the AlGaNlayer in the second superlattice layer is 0.5 nm or greater and 10 nm orsmaller, a thickness of the GaN layer in the second superlattice layeris 0.5 nm or greater and 5 nm or smaller, and a thickness of the p-typeGaN-based layer is 5 nm or greater and 50 nm or smaller.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, eachof thicknesses of the AlGaN layer and the GaN layer in the firstsuperlattice layer is 1.5 nm or greater and 5 nm or smaller, a thicknessof the quantum well layer in the multiple quantum well layer is 1 nm orgreater and 2 nm or smaller, a thickness of the quantum barrier layer inthe multiple quantum well layer is 6 nm or greater and 20 nm or smaller,a thickness of the AlGaN layer in the second superlattice layer is 1 nmor greater and 6 nm or smaller, and a thickness of the GaN layer in thesecond superlattice layer is 0.5 nm or greater and 3 nm or smaller.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, athickness of the first GaN-based layer is 1500 nm or greater and 3000 nmor smaller, a thickness of the n-type GaN-based layer is 1000 nm orgreater and 2000 nm or smaller, each of thicknesses of the AlGaN layerand the GaN layer in the first superlattice layer is 1.5 nm or greaterand 5 nm or smaller, a thickness of the second GaN-based layer is 20 nmor greater and 40 nm or smaller, a thickness of the quantum well layerin the multiple quantum well layer is 1 nm or greater and 2 nm orsmaller, a thickness of the quantum barrier layer in the multiplequantum well layer is 6 nm or greater and 20 nm or smaller, a thicknessof the AlGaN layer in the second superlattice layer is 1 nm or greaterand 6 nm or smaller, a thickness of the GaN layer in the secondsuperlattice layer is 0.5 nm or greater and 3 nm or smaller, and athickness of the p-type GaN-based layer is 10 nm or greater and 40 nm orsmaller.

The gallium nitride-based compound semiconductor according to theseaspects of the present invention can be manufactured through an MOCVDmethod. According to another aspect of the present invention, it ispreferable that a manufacturing method comprises the steps of formingthe buffer layer on the substrate at a temperature of 450° C. or higherand 600° C. or lower, sequentially forming the first GaN-based layer,the n-type GaN-based layer, and the first superlattice layer on thebuffer layer at a temperature of 1050° C. or higher and 110° C. orlower, sequentially forming the second GaN-based layer and the multiplequantum well layer on the first superlattice layer at a temperature of800° C. or higher and 900° C. or lower, and sequentially forming thesecond superlattice layer and the p-type GaN-based layer on the multiplequantum well layer at a temperature of 950° C. or higher and 1025° C. orlower.

According to another aspect of the present invention, there is provideda gallium nitride-based compound semiconductor device comprising asubstrate, an n-type AlGaN layer which is formed above the substrate, amultiple quantum well layer which is formed above the n-type AlGaN layerand in which a GaN-based quantum well layer and a GaN-based quantumbarrier layer are alternately layered, and a p-type AlGaN layer which isformed above the multiple quantum well layer.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, acompositional ratio of Al in the GaN-based quantum barrier layer in themultiple quantum well layer is larger than compositional ratios of Al inthe n-type AlGaN layer and the p-type AlGaN layer.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, abuffer layer, a first GaN-based layer, and an n-type GaN-based layer areprovided between the substrate and the n-type AlGaN layer, a secondGaN-based layer is provided between the n-type AlGaN layer and themultiple quantum well layer, and a p-type GaN-based layer is providedabove the p-type AlGaN layer.

According to another aspect of the present invention, it is preferablethat, in the gallium nitride-based compound semiconductor device, eachof compositional ratios of Al in the n-type AlGaN layer and in thep-type AlGaN layer is 5% or greater and 25% or smaller, a compositionalratio of In in the InGAN quantum well layer or the AlInGaN quantum welllayer in the multiple quantum well layer is 3% or greater and 20% orsmaller, a compositional ratio of Al in the AlInGaN quantum barrierlayer or the AlGaN quantum barrier layer in the multiple quantum welllayer is 1% or greater and 30% or smaller, and a band gap of the quantumwell layer is smaller than a band gap of the quantum barrier layer.

The gallium nitride-based compound semiconductor device according tothese aspects of the present invention can be manufactured through anMOCVD method. According to another aspect of the present invention, itis preferable that a manufacturing method comprises the steps of formingthe buffer layer on the substrate at a temperature of 450° C. or higherand 600° C. or lower, sequentially forming the first GaN-based layer,the n-type GaN-based layer, and the n-type AlGaN layer on the bufferlayer at a temperature of 1050° C. or higher and 1100° C. or lower,sequentially forming the second GaN-based layer and the multiple quantumwell layer on the n-type AlGaN layer at a temperature of 800° C. orhigher and 900° C. or lower, and sequentially forming the p-type AlGaNlayer and the p-type GaN-based layer on the multiple quantum well layerat a temperature of 950° C. or higher and 1025° C. or lower.

According to another aspect of the present invention, it is preferablethat the gallium nitride-based compound semiconductor device furthercomprises an n electrode which is connected to the n-type GaN-basedlayer, a p electrode which is connected to the p-type GaN-based layer,and a power supply which applies a voltage between the n electrode andthe p electrode. According to another aspect of the present invention,it is preferable that the gallium nitride-based compound semiconductordevice is used as a light source to realize a device which irradiateslight having a wavelength of 400 nm or shorter. Because a device inwhich the gallium nitride-based compound semiconductor device accordingto the present invention is incorporated as a light source has asuperior light emission efficiency at a short wavelength band(wavelength of 400 nm or shorter), such a device is applicable for useswhich require a light source of a short wavelength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a structure of a gallium nitride-basedcompound semiconductor device.

FIG. 2 is a diagram showing an electron microscope photograph of a crosssection of a light emitting element.

FIG. 3 is a diagram showing another electron microscope photograph of across section of a light emitting element.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be describedreferring to the drawings.

First Preferred Embodiment

FIG. 1 shows a structure of a light emitting element (LED) which is aGaN-based compound semiconductor according to a first preferredembodiment of the present invention. The light emitting element ismanufactured by growing a plurality of layers on a substrate through anMOCVD method (Metal Organic Chemical Vapor Deposition). Morespecifically, the light emitting element is manufactured through thefollowing processes. First, an MOCVD device will be briefly described,although the MOCVD device itself is well known in the art. A susceptorand a gas introduction tube are provided within a reaction tube. Asubstrate is placed on the susceptor and a material gas is suppliedwhile the substrate is heated by a heater to let a reaction occur on thesubstrate. The gas introduction section is provided, for example, at twolocations of the reaction tube. A material gas such as trimethyl galliumand silane gas is introduced from one location from a lateral directionof the substrate and mixture gas of hydrogen and nitrogen or the like issupplied from the other location via a permeable microporous member fromabove the substrate.

A sapphire c-surface substrate 10 is prepared, set on the susceptor ofan MOCVD device at a normal pressure, and thermally treated in ahydrogen atmosphere for 10 minutes with a substrate temperature of 1100°C. Then, the temperature of the substrate 10 is reduced to 500° C. andan SiN buffer film 12 is discontinuously formed on the substrate byflowing monomethyl silane gas and ammonia gas for 100 seconds. Thediscontinuous SiN film 12 is provided in order to reliably reducedislocations within the layer, but may be omitted in the presentembodiment. Next, trimethyl gallium and ammonia gas are supplied whilethe temperature is maintained at 500° C. to form a GaN buffer layer 14to a thickness of 25 nm. The GaN buffer layer 14 functions as a lowtemperature buffer layer. Then, the temperature is raised to 1075° C.and trimethyl gallium and ammonia gas are again supplied to form anundoped GaN layer 16 to a thickness of 2 μm. Monomethyl silane gas isadded to the trimethyl gallium and ammonia gas to form a Si-doped n-GaNlayer 18 to a thickness of 1 μm. The monomethyl silane gas is added todope Si into GaN to form an n-type layer and a carrier density withinthe Si-doped n-GaN layer 18 is approximately 5×10¹⁸ cm^(−3.)

Then, 50 pairs of Si-doped n-Al_(0.1)Ga_(0.9)N (2 nm) and Si-doped n-GaN(2 nm) are alternately formed while the temperature of the substrate ismaintained at 1075° C. to form an SLS (Strained Layer Superlattice)layer 20. AlGaN can be formed by supplying trimethyl aluminum inaddition to the trimethyl gallium and ammonia gas (in practice,monomethyl silane gas is also supplied in order to dope Si). An averagecarrier density within the SLS layer 20 is 5×10¹⁸ cm⁻³.

Next, the substrate temperature is lowered to 830° C. and an undoped GaNlayer 22 is formed to a thickness of 30 nm. Then, 7 pairs of undopedIn_(0.05)Ga_(0.95)N (1.5 nm) and undoped Al_(0.1)In_(0.02)Ga_(0.88)N(9.5 nm) are alternately layered to form an MQW (multiple quantum well)light emitting layer 24. In this process, the InGaN layer is formed bysupplying trimethyl gallium, trimethyl indium, and ammonia gas and theAlInGaN layer is formed by supplying trimethyl aluminum in addition tothe trimethyl gallium, trimethyl indium, and ammonia gas. The MQW lightemitting layer 24 comprises well layers and barrier layers which arealternately layered. The undoped InGaN layer functions as the well layerand the undoped AlInGaN layer functions as the barrier layer. A band gapof the well layer is set to be smaller than the band gap of the barrierlayer. In the present embodiment, the well layer is InGaN layer, but itis also possible to form the well layer with AlInGaN. When both the welllayer and the barrier layer are to be formed with AlInGaN, the Alcompositions of the layers are adjusted such that the band gap of thewell layer is smaller than the band gap of the barrier layer. Thecompositional ratio of Al in the well layer is preferably 0%-20%. TheInGaN well layer of FIG. 1 represents a configuration in which thecompositional ratio of Al is 0%. The compositional ratio of Al in thebarrier layer is preferably 1%-30%.

After the MQW light emitting layer 24 is formed, the temperature of thesubstrate 10 is raised to 975° C. and 50 pairs of Mg-doped p-AlGaN (1.5nm) and Mg-doped p-GaN (0.8 nm) are alternately formed to form a p-SLSlayer 26. Then, a p-GaN layer 28 is formed to a thickness of 15 nm.Carrier concentrations in the p-SLS layer 26 and p-GaN layer 28 are5×10¹⁷ cm⁻³ and 3×10¹⁸ cm⁻³, respectively.

The SLS layer 20 and the SLS layer 26 formed to sandwich the lightemitting layer 24 function as clad layers for confining carriers.

After an LED wafer is manufactured through the above-describedprocesses, the LED wafer is taken out from the MOCVD device, Ni (10 nm)and Au (10 nm) are sequentially vacuum-evaporated and layered on thesurface, and a thermal treatment is applied in a nitrogen gas atmospherecontaining 5% of oxygen at a temperature of 520° C. so that the metalfilm becomes a p transparent electrode 30. After the p electrode 30 isformed, a photoresist is applied on the surface, the p electrode 30 isetched using the photoresist as an etching mask to expose the n-GaNlayer 18, Ti (5 nm) and Al (5 nm) are evaporated on the exposed n-GaNlayer 18, and a thermal treatment is applied in a nitrogen gas for 30minutes at a temperature of 450° C. to form an n electrode 32.

In FIG. 1, the SLS layer 20 corresponds to a first superlattice layer,the MQW light emitting layer 24 corresponds to a multiple quantum welllayer, and the SLS layer 26 corresponds to a second superlattice layer.Furthermore, the undoped GaN layer 16 corresponds to a first GaN-basedlayer, the Si-doped n-GaN layer 18 corresponds to an n-type GaN-basedlayer, the undoped GaN layer 22 corresponds to a second GaN-based layer,and the p-GaN layer 28 corresponds to a p-type GaN-based layer.

Although not shown in FIG. 1, an Au pad for wire bonding having athickness of 500 nm is formed on a part of the p electrode 30 and nelectrode 32, a rearside surface of the substrate 10 is ground to athickness of 100 μm, and chips are cut through scrubbing and are mountedto manufacture light emitting element devices (LED devices).

Table 1 shows a material, a range of composition, and a range of carrierconcentration for each layer shown in FIG. 1. TABLE 1 RANGE OF CARRIERCON- RANGE OF CENTRATION MATERIAL OF LAYER COMPOSITION (1/cm³) p⁺GaN1-50E17 Mg-DOPED p-AlGaN/ Al = 5-25% 1-30E17 Mg-DOPED p-GaN SLSAlInGaN/AlInGaN MQW AlInGaN (WELL) UNDOPED (In = 3-20%, Al = 0-20%),AlInGaN (BARRIER) (Al = 1-20%, In = 0-10%) Compositions are selectedsuch that a band gap of the well layer is smaller than a band gap of thebarrier layer. UNDOPED GaN UNDOPED Si-DOPED n-AlGaN/ Al = 5-25% 1-8E18Si-DOPED n-GaN SLS Si-DOPED n-GaN 1-8E18 UNDOPED GaN UNDOPED BUFFERLAYER UNDOPED SiN BUFFER (may be omitted) SAPPHIRE C-SURFACE SUBSTRATE

In Table 1, the MQW light emitting layer 24 is shown as AlInGaN/AlInGaNMQW layer. In this representation, the former represents the well layerand the latter represents the barrier layer. In Table 1, it is importantto note that the compositional ratios of Al in SLS layers 20 and 26 are5% or greater and 25% or smaller, the compositional ratio of Al in thewell layer of the MQW light emitting layer 24 is 0% or greater and 20%or smaller, the compositional ratio of In in the well layer is 3% orgreater and 20% or smaller, the compositional ratio of Al in the barrierlayer of the MQW light emitting layer 24 is 1% or greater and 20% orsmaller, and the compositional ratio of In in the barrier layer is 0% orgreater and 10% or smaller. Alternatively, the compositional ratio of Alin the barrier layer of the MQW light emitting layer 24 may be set to 1%or greater and 30% or smaller. Regarding a relationship between thecomposition of Al in the barrier layer and the compositions of Al in theSLS layer 20 and SLS layer 26, it is preferable that the compositionalratio of Al in the barrier layer is greater than the compositional ratioof Al in the SLS layers 20 and 26. Electrons and holes which arecarriers recombine in the well layer of the MQW light emitting layer 24and light is emitted. When the compositional ratio of Al in the barrierlayer is increased, the band gap is increased, and consequently, thecarriers can be efficiently confined in the well layer of the MQW lightemitting layer 24 and the light emission efficiency can be improved.When the compositional ratio of Al in the barrier layer is increased,the effective band gap of the well layer of the MQW light emitting layer24 is also increased.

Because the compositional ratio of Al of the well layer includes 0% andthe compositional ratio of In in the barrier layer includes 0%, thefollowing four combinations are possible as the materials for the MQWlight emitting layer 24:

(a) InGaN well layer and AlGaN barrier layer;

(b) InGaN well layer and AlInGaN barrier layer;

(c) AlInGaN well layer and AlGaN barrier layer; and

(d) AlInGaN well layer and AlInGaN barrier layer.

In any of these combinations, the compositions are selected such thatthe band gap of the well layer is smaller than the band gap of thebarrier layer. FIG. 1 shows a configuration of the combination (b).

Table 2 shows preferred thicknesses for the layers. TABLE 2 MATERIAL OFLAYER PREFERRED THICKNESS p⁺GaN 5-50 nm Mg-DOPED p-AlGaN/ AlGaN (0.5-10nm)/ Mg-DOPED p-GaN SLS GaN (0.5-5 nm) (20-100 PAIRS) AlInGaN/AlInGaNMQW AlInGaN (WELL) (1-5 nm)/ AlInGaN (BARRIER) (2-50 nm) UNDOPED GaN5-100 nm Si-DOPED n-AlGaN/ AlGaN (1-10 nm)/GaN (1-10 nm) Si-DOPED n-GaNSLS (10-500 PAIRS) Si-DOPED n-GaN 500-10000 nm UNDOPED GaN 500-3000 nmBUFFER LAYER 10-40 nm SiN BUFFER (may be omitted) SAPPHIRE C-SURFACESUBSTRATE

As shown in Table 2, regarding the MQW light emitting layer 24, thethickness of the AlInGaN (or InGaN) well layer is 1 nm or greater and 5nm or smaller and the thickness of the AlInGaN (or AlGaN) barrier layeris 2 nm or greater and 50 nm or smaller. The thickness of AlGaN in theSLS layer 20 is 1 nm or greater and 10 nm or smaller and the thicknessof GaN in the SLS layer 20 is 1 nm or greater and 10 nm or smaller. Thethickness of AlGaN in the SLS layer 26 is 0.5 nm or greater and 10 nm orsmaller and the thickness of GaN in the SLS layer 26 is 0.5 nm orgreater and 5 nm or smaller.

Regarding the other layers, the thickness of the GaN buffer layer 14 is10 nm or greater and 40 nm or smaller, the thickness of the undoped GaNlayer 16 is 500 nm or greater and 3000 nm or smaller (preferably 500 nmor greater and 2000 nm or smaller), the thickness of the Si-doped n-GaNlayer 18 is 500 nm or greater and 10000 nm or smaller, the thickness ofthe undoped GaN layer 22 is 5 nm or greater and 100 nm or smaller, andthe thickness of the p-GaN layer 28 is 5 nm or greater and 50 nm orsmaller.

Table 3 shows more preferable thicknesses of these layers. TABLE 3MATERIAL OF LAYER MORE PREFERRED THICKNESS p⁺GaN 10-40 nm Mg-DOPEDp-AlGaN/Mg-DOPED AlGaN (1-6 nm)/GaN (0.5-3 nm) p-GaN SLS (40-60 PAIRS)AlInGaN/AlInGaN MQW AlInGaN (WELL) (1-2 nm)/ AlInGaN (BARRIER) (6-20 nm)(5-10 PAIRS) UNDOPED GaN 20-40 nm Si-DOPED n-AlGaN/ AlGaN (1.5-5 nm)/GaN(1.5-5 nm) Si-DOPED n-GaN SLS (40-60 PAIRS) Si-DOPED n-GaN 1000-2000 nmUNDOPED GaN 1500-3000 nm BUFFER LAYER 25-35 nm SiN BUFFER (may beomitted) SAPPHIRE C-SURFACE SUBSTRATE

As shown in Table 3, regarding the MQW light emitting layer 24, thethickness of the AlInGaN (or InGaN) well layer is 1 nm or greater and 2nm or smaller and the thickness of the AlInGaN (or AlGaN) barrier layeris 6 nm or greater and 20 nm or smaller. The thickness of AlGaN in theSLS layer 20 is 1.5 nm or greater and 5 nm or smaller and the thicknessof GaN in the SLS layer 20 is 1.5 nm or greater and 5 nm or smaller. Thethickness of AlGaN in the SLS layer 26 is 1 nm or greater and 6 nm orsmaller and the thickness of GaN in the SLS layer 26 is 0.5 nm orgreater and 3 nm or smaller. Regarding the other layers, the thicknessof the GaN buffer layer 14 is 25 nm or greater and 35 nm or smaller, thethickness of the undoped GaN layer 16 is 1500 nm or greater and 3000 nmor smaller (preferably, 1500 nm or greater and 2000 nm or smaller), thethickness of the Si-doped n-GaN layer 18 is 1000 nm or greater and 2000nm or smaller, the thickness of the undoped GaN layer 22 is 20 nm orgreater and 40 nm or smaller, and the thickness of the p-GaN layer 28 is10 nm or greater and 40 nm or smaller.

Table 4 shows growth temperatures when the layers are grown using theMOCVD device. TABLE 4 MATERIAL OF LAYER TEMPERATURE (° C.) p⁺GaN 950-1025 Mg-DOPED p-AlGaN/Mg-DOPED p-GaN SLS  950-1025 InGaN/AlInGaNMQW 800-900 UNDOPED GaN 800-900 Si-DOPED n-AlGaN/ 1050-1100 Si-DOPEDn-GaN SLS Si-DOPED n-GaN 1050-1100 UNDOPED GaN 1050-1100 BUFFER LAYER450-600 SiN BUFFER 450-600 SAPPHIRE C-SURFACE SUBSTRATE

As shown in Table 4, the GaN buffer layer 14 is grown at a temperatureof 450° C. or higher and 600° C. or lower, the undoped GaN layer 16 isgrown at a temperature of 1050° C. or higher and 1100° C. or lower, theSi-doped n-GaN layer 18 is grown at a temperature of 1050° C. or higherand 1100° C. or lower, the SLS layer 20 or a single layer of the AlGaNlayer 20 is grown at a temperature of 1050° C. or higher and 1100° C. orlower, the undoped GaN layer 22 is grown at a temperature of 800° C. orhigher and 900° C. or lower, the MQW light emitting layer 24 is grown ata temperature of 800° C. or higher and 900° C. or lower, the SLS layer26 or a single layer of the AlGaN layer 26 is grown at a temperature of950° C. or higher and 1025° C. or lower, and the p-GaN layer 28 is grownat a temperature of 950° C. or higher and 1025° C. or lower.

A light emitting element thus manufactured was introduced into anintegrating sphere, the p electrode 30 and the n electrode 32 wereconnected to a power supply, a current was supplied, and a total powerof light emitted from the chip was measured. A light power ofapproximately 2 mW was obtained at a supplied current of 20 mA. Thelight emission wavelength was confirmed to be within a wavelength of 372nm±5 nm, although the light emission wavelength slightly varies on thewafer surface of a diameter of 2 inches. The external quantum efficiencywas approximately 3%.

The present inventors formed a number of wafers with different materialsand thicknesses for the layers shown in FIG. 1, performed similarevaluations, and found that the restrictive conditions for the layersdiffer depending on the light emission wavelength. More specifically,when the light emission peak wavelength is 380 nm-400 nm, a light powerof 1 mW or greater can be obtained when the thicknesses of the layersare in the ranges of preferred thicknesses as shown in Table 2.

When, on the other hand, the light emission peak wavelength is 365nm-380 nm and the thicknesses of the layers are deviated from the rangesof preferred thicknesses as shown in Table 2, the light power is rapidlyreduced to 0.1 mW or less and the light power became 1 mW or less whenthe thicknesses of the layers are deviated from the ranges of preferredthicknesses shown in Table 3. From these results, it was confirmed thatthe light emission efficiency in the short wavelength band (wavelengthof 400 nm or shorter) can be improved by setting the thicknesses withinthe range shown in Table 2 or Table 3.

A reason why the light emission efficiency can be improved may beconsidered as follows. FIG. 2 is an explanatory diagram showing anelectron microscope photograph of a cross section of a light emittingelement manufactured in the present embodiment. Upon detailedobservation of the explanatory diagram of the electron microscopephotograph of cross section, it can be seen that the dislocations arereduced at an interface between the n-GaN/AlGaN SLS layer 20 and theupdoped GaN layer 22. It can be considered that because of distortionscontained in the SLS layer 20 and the change in the growth temperatureat this boundary, dislocations which are transferred from the lowerlayers are bent in the lateral direction and the dislocations in the MQWlight emitting layer 24 formed immediately above are reduced. In otherwords, it can be considered that the dislocations in the MQW lightemitting layer 24 can be inhibited by adjusting the compositions andthicknesses of the layers in the above-described ranges.

Another reason why the light emission efficiency can be improved may bethe compositional fluctuation of the light emitting layer 24. In the MQWlight emitting layer 24 in which the AlInGaN (or InGaN) well layer andAlInGaN (or AlGaN) barrier layer are layered, the composition of In islow and compositional fluctuation of InGaN does not tend to occur. Whencompositional fluctuation occurs, light is emitted at a local region inwhich the composition of In is large, and, thus, the injected carriersare captured at the local region and cannot reach the dislocations. As aresult, the efficiency is not reduced. However, because Al is added tothe barrier layer, it can be considered that, by simultaneouslyincreasing compositions of In and Al, the composition of In and Al canbe increased while maintaining the light emission wavelength at a shortwavelength, and, as a result, the compositional fluctuation isincreased. FIG. 3 is an explanatory diagram showing an electronmicroscope photograph of a cross section. In FIG. 3, it can be seen thatthe wall layer of the light emitting layer 24 is uneven. In other words,it can be considered that, by adjusting the materials and compositionalratios in the MQW light emitting layer 24 in the above-described ranges,the compositional fluctuation can be increased and reduction in thelight emission efficiency due to dislocations can be inhibited.

In this manner, by employing a layer structure as shown in FIG. 1 andsetting the thicknesses of the layers to the ranges shown in Table 2 orTable 3, it is possible to obtain a high light emission efficiency.

The present inventors have found that it is possible to obtain a highlight emission efficiency at a wavelength of 400 nm or shorter similarto the above by employing a single layer of AlGaN in place of the SLSlayer 20 and a single layer of AlGaN in place of the SLS layer 26. Morespecifically, an AlGaN layer having a composition of Al of 5%-25% isformed to a thickness of 50 nm or greater and 500 nm or smaller, morepreferably, 70 nm or greater and 300 nm or smaller in place of the SLSlayer 20 and an AlGaN layer having a composition of Al of 5%-25% isformed to a thickness of 50 nm or greater and 500 nm or smaller, morepreferably, 70 nm or greater and 200 nm or smaller in place of the SLSlayer 26. The overall structure of the configuration in which singlelayers of AlGaN are used is: sapphire substrate 10/discontinuous film ofSiN 12/GaN buffer layer 14/undoped GaN layer 16/Si-doped n-GaN layer18/AlGaN layer 20/undoped GaN layer 22/MQW light emitting layer 24/AlGaNlayer 26/p-GaN layer 28.

In this configuration also, the compositional ratios of Al are selectedsuch that the band gap of the well layer of the MQW 24 is smaller thanthe band gap of the barrier layer. Moreover, it is preferable that thecompositional ratio of Al in the barrier layer of the MQW 24 is selectedto be larger than the compositional ratio of Al in the AlGaN layers 20and 26. Specifically, it is preferable to set the compositional ratio ofAl in the barrier layer of MQW 24 to approximately 30%.

The present inventors also manufactured a light emitting element inwhich the SiN film 12 shown in FIG. 1 is not formed, a GaN-based layer16 having a structure of GaN layer/SiN layer/GaN layer with SiN insertedin the GaN layer is employed in place of the undoped GaN layer 16, 50pairs of undoped AlGaN (1.7 nm) and Si-doped GaN (1.7 nm) arealternately formed as the SLS layer 20, an AlGaN layer (26 nm) isemployed in place of the undoped GaN layer 22, 3 pairs of InGaN welllayer (1.7 nm) and AlGaN barrier layer (13 nm) are alternately formed asthe MQW light emitting layer 24, and 50 pairs of AlGaN (1.1 nm) and GaN(0.5 nm) are alternately formed as the SLS layer 26 and found that thelight emission efficiency is high at a wavelength of 400 nm or shorter,similar to the above-described configuration. By inserting SiN in theGaN layer, dislocations in the GaN layer formed on the SiN layer arereduced and dislocations in the MQW light emitting layer 24 can beinhibited.

Table 5 shows a material, a range of composition, and a range of carrierconcentration for each of the layers. TABLE 5 RANGE OF CARRIER CON-RANGE OF CENTRATION MATERIAL OF LAYER COMPOSITION (cm⁻³) p⁺GaN (17 nm)1-50E17 AlGaN (1.1 nm)/ Al = 5-25% 1-30E17 GaN (0.5 nm) (50 PAIRS) InGaN(WELL) (1.7 nm)/ InGaN (WELL) UNDOPED AlGaN (BARRIER) (In˜5.5%, AlGaN(13 nm) (3 PAIRS) (BARRIER) (Al˜30%) AlGaN (26 nm) Al˜20% UNDOPEDUNDOPED AlGaN (1.7 nm)/ Al˜18% 1-8E18 Si-DOPED n-GaN (1.7 nm) SLS, (50PAIRS) Si-DOPED n-GaN 1-8E18 UNDOPED GaN2 (800-1200 nm) UNDOPED SiN HIGHTEMPERATURE BUFFER UNDOPED GaN1 (800-1200 nm) UNDOPED BUFFER LAYER (˜25nm) UNDOPED SAPPHIRE C-SURFACE SUBSTRATE

The compositional ratio of Al in the AlGaN barrier layer of the MQWlight emitting layer 24 is 30% which is larger than the compositionalratio of Al of the SLS layer 20 which is approximately 18% and is largerthan the compositional ratio of Al in the SLS layer 26 which is 5%-25%.When the composition of Al in AlGaN is increased, the crystal quality isdegraded, and therefore, it is preferable that the upper limit of thecomposition of Al in the AlGaN barrier layer of the MQW light emittinglayer 24 is approximately 30%.

The light emitting element according to the present embodiment has ahigh light emission efficiency at a wavelength of 400 nm or shorter, andthus, it is possible to manufacture various products using thischaracteristic. In the following description, example devices in whichthe light emitting element shown in FIG. 1 is used as a light sourcewill be described.

Second Preferred Embodiment

When texts, objects, or the like are drawn by a commercially availableblack pen (fluorescent pen), the drawn objects cannot be seen undervisible light illumination, but can be seen when an ultraviolet ray isirradiated onto the drawn objects. Although a color black pen (a colorobject appears when ultraviolet ray is irradiated) is also commerciallyavailable, in order to reproduce colors, the wavelength of theultraviolet ray to be irradiated must be 400 nm or shorter, moreprecisely, 380 nm or shorter. In the related art, a light source such asa fluorescence black light or a mercury lamp is used. However, theselight sources are large and consume large power and there is a problemin that a large-scale power supply is necessary.

When the light emitting element device (LED) as shown in FIG. 1 is usedas a light source for reproducing the drawn objects, the device is smalland can be driven by a battery. Objects drawn by a black pen werereproduced by irradiating light with LEDs having peak wavelengths of 400nm, 385 nm, and 372 nm. The intensities of illuminated light wereapproximately 5 mW (400 nm), 3 mW (385 nm), and 1 mW (372 nm).

In the case of the 400 nm LED, although the objects appeared, the colorswere not reproduced and the intensity of fluorescence was a very lowlevel which can barely be seen. When light is irradiated by the 385 nmLED, a strong intensity of fluorescence which allows clear display ofthe shape of the objects was obtained, but the colors were notreproduced. The color reproducibility for red was particularly inferior.In the case of the LED of a wavelength of 372 nm, on the other hand,although the irradiation intensity was 1 mW, which is low, thefluorescence was strong and allowed the objects to be seen in a brightroom and the three primary colors were accurately reproduced.

From these results, it was confirmed that the LED having a wavelengthband of 365 nm-380 nm was suitable as a light source for reproducingobjects drawn by a black pen (fluorescent pen) which is commerciallyavailable at low cost. By incorporating the LED of the presentembodiment along with a battery into products such as a key holder, ablack pen, and an eraser, it is possible to obtain a system for drawinginvisible text and drawings which can be easily reproduced.

Third Preferred Embodiment

The LED according to the present embodiment was irradiated onto the skinof a human body for a short period of time and the effects of theirradiation were observed. Light was irradiated from LEDs having peakwavelengths of 400 nm (5 mW), 385 nm (3 mW), and 372 nm (1 mW) onto theskin for 10 minutes and changes in the skin (commonly referred to as“tanning”) were observed. As a result, it was found that in the case ofthe LED of peak wavelength of 400 nm (5 mW), there was almost no effectwhile there was some change in the case of the LED of 385 nm (3 mW). Inthe case of the LED of 372 nm (1 mW), on the other hand, there wassignificant tanning. These results show that the LED in the wavelengthband of 365 nm-380 nm causes tanning of the human body. Tanning deviceswere manufactured using the LED as a light source. A tanning devicewhich allows tanning of a spot of a diameter of 5 mm and a tanningdevice in which LEDs were placed on a straight line having a length of 3cm to allow tanning of line shape were manufactured and experiments wereperformed. Both devices cause tanning by irradiating light for 10minutes. It was also found that when the light is irradiated for 30minutes or longer, the skin was damaged.

In the related art, because tanning devices use ultraviolet lamps, thesetanning devices were well suited for usage with a large irradiationarea, but not for tanning of only a small region, and required, forexample, that the parts other than the parts for which tanning isdesired be covered by a towel or the like. The tanning device of thepresent embodiment, on the other hand, can create arbitrary tanning ofspots and lines.

Fourth Preferred Embodiment

Light was irradiated onto skin to which a commercially available UV-cutcosmetic (SPF50+PA+++) was applied by a tanning device of the thirdpreferred embodiment in which an LED having a peak wavelength of 372 nm(1 mW) was equipped as the light source. As a result, it was confirmedthat the degree of tanning was very small compared to the case in whichthe UV-cut cosmetic is not applied. As described, the LED of the presentembodiment can be used as a device for evaluating performance of anUV-cut cosmetic.

In the related art, this type of examination device was large and abroad skin surface is required for observing the effects. As describedin the above-described preferred embodiments, the examination device ofthe present embodiment can cause tanning of an arbitrary shape of a spotor line or an arbitrary part. Therefore, the examination device of thepresent embodiment allows observation of degree of tanning for each partof a human body or to observe long-term irradiation effects by carryingthe examination device.

Fifth Preferred Embodiment

A light accumulating material is used in a dial plate of a clock andsigns such as evacuation guides or the like. This configuration takesadvantage of a mechanism where when light is irradiated onto the lightaccumulating agent, the fluorescence continues even after the light isextinguished, to allow text or the like to be read in darkness. Inrecent years, the light accumulating period is elongated and the threeprimary colors can be displayed. For example, a short remaining lighttype device in which zinc sulfide and copper are combined and a longremaining light type device in which strontium aluminate and a rareearth metal are combined are known. The sensitivity of such a lightaccumulating agent is typically at a wavelength of 400 nm or shorter.Therefore, by combining the light accumulating agent and the LED of thepresent embodiment, it is possible to manufacture a display devicehaving very low power consumption in which an operation to irradiatelight for a short period of time and an operation to extinguish thelight are repeated. It is also possible to create a (non-volatile)display device for emergencies in which the display does not disappearwhen the power supply is switched off.

Display devices were manufactured by combining LEDs having peakwavelengths of 400 nm (5 mW), 385 nm (3 mW), and 372 nm (1 mW) and lightaccumulating agents of three primary colors. The light accumulatingagent was processed in a plate-like shape, light of the LED of thepresent embodiment was irradiated from the rearside surface, and thelight emission from the front surface was observed. A cycle ofirradiation of 10 minutes and termination of irradiation for apredetermined period of time was repeated. Even after the irradiationwas terminated for 30 minutes, light emission from the lightaccumulating agent was observed in a room having a brightness similar tothat of an ordinary office room. In total darkness, light emission fromthe light accumulating agent was observed even after the light wasextinguished for approximately 1 hour. Similar advantages were obtainedfor all wavelengths, and the strongest remaining light was observed whenan LED having a peak wavelength of 400 nm (5 mW) was used as the lightsource.

As described, with a display device which uses an LED of a wavelengthband of 365 nm-400 nm and a light accumulating agent, it is possible tosignificantly reduce the power consumption compared to the displaydevices of the related art.

The reproducibility of colors were also observed. Light from the LEDs ofthe peak wavelength of 400 nm (5 mW) and of 385 nm (3 mW) are observedby the naked eye in blue color-purple color. Therefore, when red lightis emitted from the light accumulating agent, the color of the LED andthe color of the light from the light accumulating agent are mixed, andpure red cannot be reproduced. This is similar to green. Light from theLED of the peak wavelength of 372 nm (1 mW), on the other hand, isalmost invisible to the naked eyes, and therefore, it is possible toaccurately reproduce three primary colors. Therefore, a display devicein which an LED of a wavelength band of 365 nm-380 nm is used as a lightsource and a light accumulating agent is used has a low powerconsumption and can achieve a full-color display.

Sixth Preferred Embodiment

A compound eye of an insect such as a moth has a peak sensitivity at awavelength of 360 nm. An extermination device for insects using anultraviolet lamp which takes advantage of this characteristic iscommercially available. An ultraviolet lamp is mounted on a street lightand an insect extermination device is placed near the ultraviolet lamp.Because the ultraviolet lamp also emits visible light, the ultravioletlamp is generally bright. There is also a problem in that the powerconsumption is high. The LED of the present embodiment was used as thelight source for attracting the insects. The peak wavelength was 372 nm.The LED is placed in darkness and gathering of insects was observed. Atthe same time, a mercury lamp of 2 W was placed at a different locationfor comparison. In order to reduce the power consumption of the LED andto increase the light emission peak intensity, the LED was driven with apulse having a peak current of 200 mA, a peak power of approximately 10mW, a pulse width of 10 mS, and a repetition frequency of 10 Hz (averagepower of 1 mW). Although the light power was smaller for the LED, it wasobserved that more insects gathered around the LED. The mercury lamp wasobserved by the naked eye as blue color-purple color, but the LED wasalmost invisible to the naked eye. From these results, it can be seenthat the LED having a wavelength band of 365 nm-380 nm can be used as alight source for collecting insects.

A device was manufactured in which a lamp of a commercially availableinsect extermination device was changed to the LED. 200 LEDs of awavelength of 372 nm were used and driven by a pulse similar to theabove-described experiment. As a result, it was possible to exterminateapproximately the same number of insects in one night compared to theoriginal device. The insect extermination device in which the LED of thepresent embodiment is used as the light source has advantages that thepower consumption is low and the degree of freedom for layout of thelight source is increased because the LED is small. In addition, becausethe light is almost invisible to the naked eye, it is possible to usethis device in an environment in which illumination is not desired.

Preferred embodiments of the present invention have been described.These preferred embodiments, however, are only exemplary, and otherdevices may be created using the LED of the present invention. Forexample, the present invention can be applied to a device for judgingpaper currency or testing for forgery of paper currency, or for cleaningair or water using a photocatalytic effect obtained by irradiating lighton titanium oxide.

In the embodiments, it is possible to use an AlGaN layer in place of theGaN layer 16 and to use an n-type AlGaN layer in place of the n-type GaNlayer 18. Moreover, in the embodiments, it is possible to use an InGaNlayer in place of the GaN layer 22 and ap-type InGaN layer in place ofthe p-type GaN layer 28. The GaN-based layer includes, in addition tothe GaN layer, An AlGaN layer and an InGaN layer in which the Ga in GaNis replaced by Al or In.

1. A gallium nitride-based compound semiconductor device comprising: asubstrate; a first superlattice layer which is formed above thesubstrate and in which an n-type AlGaN layer and an n-type GaN layer arealternately layered; a multiple quantum well layer which is formed abovethe first superlattice layer and in which a GaN-based quantum well layerand a GaN-based quantum barrier layer are alternately layered; and asecond superlattice layer which is formed above the multiple quantumwell layer and in which a p-type AlGaN layer and a p-type GaN layer arealternately layered.
 2. A gallium nitride-based compound semiconductordevice according to claim 1, wherein a buffer layer, a first GaN-basedlayer which is formed above the buffer layer, and an n-type GaN-basedlayer which is formed above the first GaN-based layer are providedbetween the substrate and the first superlattice layer; a secondGaN-based layer is provided between the first superlattice layer and themultiple quantum well layer; and a p-type GaN layer is provided abovethe second superlattice layer.
 3. A gallium nitride-based compoundsemiconductor device according to claim 2, wherein the first GaN-basedlayer has a structure in which an SiN layer is inserted in a GaN layer,and the second GaN-based layer has an AlGaN layer.
 4. A galliumnitride-based compound semiconductor device according to claim 1,wherein a compositional ratio of Al in the GaN-based quantum barrierlayer in the multiple quantum well layer is larger than compositionalratios of Al in the first superlattice layer and the second superlatticelayer.
 5. A gallium nitride-based compound semiconductor deviceaccording to claim 1, wherein each of compositional ratios of Al in theAlGaN layers in the first superlattice layer and in the secondsuperlattice layer is 5% or greater and 25% or smaller; a compositionalratio of In in the InGaN quantum well layer or the AlInGaN quantum welllayer in the multiple quantum well layer is 3% or greater and 20% orsmaller; a compositional ratio of Al in the AlGaN quantum barrier layeror the AlInGaN quantum barrier layer in the multiple quantum well layeris 1% or greater and 30% or smaller; and a band gap of the quantum welllayer is smaller than a band gap of the quantum barrier layer.
 6. Agallium nitride-based compound semiconductor device according to claim1, wherein each of thicknesses of the AlGaN layer and the GaN layer inthe first superlattice layer is 1 nm or greater and 10 nm or smaller; athickness of the quantum well layer in the multiple quantum well layeris 1 nm or greater and 5 nm or smaller; a thickness of the quantumbarrier layer in the multiple quantum well layer is 2 nm or greater and50 nm or smaller; a thickness of the AlGaN layer in the secondsuperlattice layer is 0.5 nm or greater and 10 nm or smaller; and athickness of the GaN layer in the second super lattice layer is 0.5 nmor greater and 5 nm or smaller.
 7. A gallium nitride-based compoundsemiconductor device according to claim 2, wherein a thickness of thefirst GaN-based layer is 500 nm or greater and 3000 nm or smaller; athickness of the n-type GaN-based layer is 500 nm or greater and 10000nm or smaller; each of thicknesses of the AlGaN layer and the GaN layerin the first superlattice layer is 1 nm or greater and 10 nm or smaller;a thickness of the second GaN-based layer is 5 nm or greater and 100 nmor smaller; a thickness of the quantum well layer in the multiplequantum well layer is 1 nm or greater and 5 nm or smaller; a thicknessof the quantum barrier layer in the multiple quantum well layer is 2 nmor greater and 50 nm or smaller; a thickness of the AlGaN layer in thesecond superlattice layer is 0.5 nm or greater and 10 nm or smaller; athickness of the GaN layer in the second superlattice layer is 0.5 nm orgreater and 5 nm or smaller; and a thickness of the p-type GaN-basedlayer is 5 nm or greater and 50 nm or smaller.
 8. A galliumnitride-based compound semiconductor device according to claim 1,wherein each of thicknesses of the AlGaN layer and the GaN layer in thefirst superlattice layer is 1.5 nm or greater and 5 nm or smaller; athickness of the quantum well layer in the multiple quantum well layeris 1 nm or greater and 2 nm or smaller; a thickness of the quantumbarrier layer in the multiple quantum well layer is 6 nm or greater and20 nm or smaller; a thickness of the AlGaN layer in the secondsuperlattice layer is 1 nm or greater and 6 nm or smaller, and athickness of the GaN layer in the second superlattice layer is 0.5 nm orgreater and 3 nm or smaller.
 9. A gallium nitride-based compoundsemiconductor device according to claim 2, wherein a thickness of thefirst GaN-based layer is 1500 nm or greater and 3000 nm or smaller; athickness of the n-type GaN-based layer is 1000 nm or greater and 2000nm or smaller; each of thicknesses of the AlGaN layer and the GaN layerin the first superlattice layer is 1.5 nm or greater and 5 nm orsmaller; a thickness of the second GaN-based layer is 20 nm or greaterand 40 nm or smaller; a thickness of the quantum well layer in themultiple quantum well layer is 1 nm or greater and 2 nm or smaller; athickness of the quantum barrier layer in the multiple quantum welllayer is 6 nm or greater and 20 nm or smaller; a thickness of the AlGaNlayer in the second superlattice layer is 1 nm or greater and 6 nm orsmaller; a thickness of the GaN layer in the second superlattice layeris 0.5 nm or greater and 3 nm or smaller; and a thickness of the p-typeGaN-based layer is 10 nm or greater and 40 nm or smaller.
 10. A galliumnitride-based compound semiconductor device comprising: a substrate; ann-type AlGaN layer which is formed above the substrate; a multiplequantum well layer which is formed above the n-type AlGaN layer and inwhich a GaN-based quantum well layer and a GaN-based quantum barrierlayer are alternately layered; and a p-type AlGaN layer which is formedabove the multiple quantum well layer.
 11. A gallium nitride-basedcompound semiconductor device according to claim 10, wherein a bufferlayer, a first GaN-based layer which is formed above the buffer layer,and an n-type GaN-based layer which is formed above the first GaN-basedlayer are provided between the substrate and the n-type AlGaN layer; asecond GaN-based layer is provided between the n-type AlGaN layer andthe multiple quantum well layer; and a p-type GaN-based layer isprovided above the p-type AlGaN layer.
 12. A gallium nitride-basedcompound semiconductor device according to claim 10, wherein acompositional ratio of Al in the GaN-based quantum barrier layer in themultiple quantum well layer is larger than compositional ratios of Al inthe n-type AlGaN layer and the p-type AlGaN layer.
 13. A galliumnitride-based compound semiconductor device according to claim 10,wherein each of compositional ratios of Al in the n-type AlGaN layer andin the p-type AlGaN layer is 5% or greater and 25% or smaller; acompositional ratio of In in the InGaN quantum well layer or the AlInGaNquantum well layer in the multiple quantum well layer is 3% or greaterand 20% or smaller; a compositional ratio of Al in the AlInGaN quantumbarrier layer or the AlGaN quantum barrier layer in the multiple quantumwell layer is 1% or greater and 30% or smaller, and a band gap of thequantum well layer is smaller than a band gap of the quantum barrierlayer.
 14. A gallium nitride-based compound semiconductor deviceaccording to claim 10, wherein a thickness of the n-type AlGaN layer is50 nm or greater and 500 nm or smaller; a thickness of the quantum welllayer in the multiple quantum well layer is 1 nm or greater and 5 nm orsmaller; a thickness of the quantum barrier layer in the multiplequantum well layer is 2 nm or greater and 50 nm or smaller; and athickness of the p-type AlGaN layer is 50 nm or greater and 500 nm orsmaller.
 15. A gallium nitride-based compound semiconductor deviceaccording to claim 11, wherein a thickness of the first GaN-based layeris 500 nm or greater and 3000 nm or smaller; a thickness of the n-typeGaN-based layer is 500 nm or greater and 10000 nm or smaller; athickness of the n-type AlGaN layer is 50 nm or greater and 500 nm orsmaller; a thickness of the second GaN-based layer is 5 nm or greaterand 100 nm or smaller; a thickness of the quantum well layer in themultiple quantum well layer is 1 nm or greater and 5 nm or smaller; athickness of the quantum barrier layer in the multiple quantum welllayer is 2 nm or greater and 50 nm or smaller; a thickness of the p-typeAlGaN layer is 50 nm or greater and 500 nm or smaller; and a thicknessof the p-type GaN-based layer is 5 nm or greater and 50 nm or smaller.16. A gallium nitride-based compound semiconductor device according toclaim 10, wherein a thickness of the n-type AlGaN layer is 70 nm orgreater and 300 nm or smaller; a thickness of the quantum well layer inthe multiple quantum well layer is 1 nm or greater and 2 nm or smaller;a thickness of the quantum barrier layer in the multiple quantum welllayer is 6 nm or greater and 20 nm or smaller; and a thickness of thep-type AlGaN layer is 70 nm or greater and 200 nm or smaller.
 17. Agallium nitride-based compound semiconductor device according to claim11, wherein a thickness of the first GaN-based layer is 1500 nm orgreater and 3000 nm or smaller; a thickness of the n-type GaN-basedlayer is 1000 nm or greater and 2000 nm or smaller; a thickness of then-type AlGaN layer is 70 nm or greater and 300 nm or smaller; athickness of the second GaN-based layer is 20 nm or greater and 40 nm orsmaller; a thickness of the quantum well layer in the multiple quantumwell layer is 1 nm or greater and 2 nm or smaller; a thickness of thequantum barrier layer in the multiple quantum well layer is 6 nm orgreater and 20 nm or smaller; a thickness of the p-type AlGaN layer is70 nm or greater and 200 nm or smaller; and a thickness of the p-typeGaN-based layer is 10 nm or greater and 40 nm or smaller.
 18. A methodfor manufacturing a gallium nitride-based compound-semiconductor deviceaccording to claim 2 through MOCVD, wherein the buffer layer is formedon the substrate at a temperature of 450° C. or higher and 600° C. orlower; the first GaN-based layer, the n-type GaN-based layer, and thefirst superlattice layer are sequentially formed on the buffer layer ata temperature of 1050° C. or higher and 1100° C. or lower; the secondGaN-based layer and the multiple quantum well layer are sequentiallyformed on the first superlattice layer at a temperature of 800° C. orhigher and 900° C. or lower; and the second superlattice layer and thep-type GaN-based layer are sequentially formed on the multiple quantumwell layer at a temperature of 950° C. or higher and 1025° C. or lower.19. A method for manufacturing a gallium nitride-based compoundsemiconductor device according to claim 11 through MOCVD, wherein thebuffer layer is formed on the substrate at a temperature of 450° C. orhigher and 600° C. or lower; the first GaN-based layer, the n-typeGaN-based layer, and the n-type AlGaN layer are sequentially formed onthe buffer layer at a temperature of 1050° C. or higher and 1100° C. orlower; the second GaN-based layer and the multiple quantum well layerare sequentially formed on the n-type AlGaN layer at a temperature of800° C. or higher and 900° C. or lower; and the p-type AlGaN layer andthe p-type GaN-based layer are sequentially formed on the multiplequantum well layer at a temperature of 950° C. or higher and 1025° C. orlower.
 20. A gallium nitride-based compound semiconductor deviceaccording to any one of claims 2 through 11, further comprising: an nelectrode which is connected to the n-type GaN-based layer; a pelectrode which is connected to the p-type GaN-based layer; and a powersupply which applies a voltage between the n electrode and the pelectrode.
 21. A device which uses a gallium nitride-based compoundsemiconductor device according to claim 20 as a light source andirradiates light having a wavelength of 400 nm or shorter.